There are two main types of mechanical stresses present during a typical inflation of an elongated balloon. Hydraulic loading on a generally cylindrical balloon wall results in hoop stress and longitudinal stress. The result of the inflation loading depends on the type of material from which the balloon walls are constructed. In simplified terms, for a compliant balloon, the load will result in the balloon adapting to the shape of surrounding constraints. For example, in a body vessel, the balloon will inflate up to contact with the vessel wall(s) and then continue to lengthen down the length of the vessel as more inflation medium is introduced into the balloon. For non-compliant balloons, the shape is defined by the configuration of the balloon walls as manufactured rather than surrounding constraints, and as inflation medium is introduced, the pressure increases while the volume remains relatively constant. When a generally cylindrical non-compliant balloon is inflated in a curved vessel, the tendency is to maintain the molded balloon shape, usually a straight shape, and thus stress the vessel walls.
Percutaneous transluminal angioplasty (PTA) balloons, which are semi-compliant to non-compliant balloons, are designed to operate at pressures between 5 to 30 atmospheres (“atm”) and do not readily conform to a curved configuration, in particular tortuous anatomies, e.g., curved body vessels. More precisely, medical angioplasty balloons when inflated in a curved configuration tend toward a straight configuration as the balloon is inflated to angioplasty pressures. This tendency to straighten is characterized herein in terms of a straightening force. As one might expect, such balloons, when extended along the vessel curvature, are impeded from straightening by the surrounding vessel. As a result, the balloon can cause unwanted straightening of, and damage to, the vessel, and/or the angioplasty balloon can kink. These balloons do not typically conform to the surrounding constraints, particularly when the surrounding constraints involve certain amounts of curvature.
Medical balloons capable of operating at angioplasty pressures yet having a low or insignificant straightening force when in a curved configuration can be useful in many applications.